Liquid crystal display devices comprising liquid crystalline compounds have widely been used for displays of watches, tabletop calculators, word processors, and others. These display devices employ the optical anisotropy and dielectric anisotropy of liquid crystalline compounds.
While liquid crystal phase includes a nematic liquid crystal phase, smectic liquid crystal phase, and cholesteric liquid crystal phase, display devices employing the nematic liquid crystal phase have most widely been used. As their display mode, dynamic scattering (DS) type, deformation of aligned phase (DAP) type, guest-host (GH) type, twisted nematic (TN) type, super twisted nematic (STN) type, and thin film transistor (TFT) type are known.
Whereas liquid crystalline compounds employed in these display modes must exhibit a liquid crystal phase at a wide temperature range having its center at room temperature, must be sufficiently stable under conditions in which the liquid crystalline compounds are used, and must have characteristics sufficient to drive the display devices, no compound which satisfies those requirements by its self has been found. Accordingly, it is an actual circumstance that several kinds to several tens kinds of liquid crystalline compounds are mixed, together with not-liquid crystalline compounds when necessary, to produce a liquid crystal composition provided with required characteristics. These liquid crystal compositions are required to be stable against moisture, light, heat, and air which usually exist under the conditions in which display devices are used, to be stable against electric field and electromagnetic radiation, and further to be chemically stable against the compounds to be mixed. The liquid crystal compositions are also required to have appropriate physical parameters such as the value of optical anisotropy (.DELTA.n) and dielectric anisotropy (.DELTA..epsilon.) depending on the display mode and the shape of display devices. Still further, it is important that each of the components in the liquid crystal compositions have good solubility to each other.
Especially, the demand for lowering threshold voltage still more which largely contributes to high speed response necessary for expanding the screen of liquid crystal display devices has been increased. For that purpose, liquid crystalline compounds having a large .DELTA..epsilon. are necessary (E. Jakeman et al., Phys. Lett., 39A. 69 (1972)).
Also, liquid crystalline compounds having a small change of threshold voltage depending on temperature are considered to be necessary.
In order to achieve these purposes, the compounds expressed by formula (a), (b), or (c) are disclosed in Laid-open Japanese Patent Publication No. Sho 55-40660, Laid-open WO Japanese Patent Publication No. Hei 2-501311, or Laid-open WO Japanese Patent Publication No. Hei 3-500413. However, .DELTA..epsilon. of these compounds is not yet sufficiently large and the change of their threshold voltage due to the change of temperature can not be said to be small. ##STR2##
Further, the compounds expressed by formula (d) or (e) are known in public through Laid-open WO Japanese Patent Publication No. Hei 3-503637 or Laid-open Japanese Patent Publication No. Hei 4-279560. However, these compounds have problems that the temperature range of liquid crystal phase is narrow or the solubility to other liquid crystal materials at low temperatures is not sufficient whereas the compounds have a large .DELTA..epsilon.. ##STR3##